This invention relates to a modulator system having a wide dynamic range and more particularly to a modulator system operable with a low source voltage.
One example of a prior art modulator system is illustrated in FIG. 1 and comprises an input terminal IN.sub.1 supplied with a signal to be modulated, another input terminal IN.sub.2 supplied with a modulation signal and an output terminal OUT for deriving out a modulated output signal. The modulator circuit further comprises transistor pairs Q.sub.1, Q.sub.2 and Q.sub.3, Q.sub.4, the emitter electrodes of respective pairs being commonly connected, driving transistor pair Q.sub.5 and Q.sub.6 respectively driving transistor pairs Q.sub.1, Q.sub.2 and Q.sub.3, Q.sub.4, and another transistor pair Q.sub.7 and Q.sub.8 constituting a constant current source for transistor pairs Q.sub.5 and Q.sub.6.
The collector electrode of transistor Q.sub.1 is connected to a source of supply V.sub.cc via a resistor R.sub.1, while the base electrode of the transistor Q.sub.1 is connected to the input terminal IN.sub.1. The collector electrode of transistor Q.sub.2 is connected to the output terminal OUT, whereas the base electrode is connected to the base electrode of transistor Q.sub.3, these base electrodes being commonly connected to a source of bias voltage E.sub.1. The collector electrode of transistor Q.sub.3 is connected directly to the collector electrode of transistor Q.sub.1, whereas the collector electrode of transistor Q.sub.4 is connected to the output terminal OUT and to the source V.sub.cc via a resistor R.sub.2. The base electrode of transistor Q.sub.4 is connected to the input terminal IN.sub.1. The collector electrode of transistor Q.sub.5 is connected to the commonly connected emitter electrodes of transistors Q.sub.1 and Q.sub.2, whereas the emitter electrode of transistor Q.sub.5 is connected to the collector electrode of transistor Q.sub.7 and to the emitter electrode of transistor Q.sub.6 via common emitter resistor R.sub.3, and base electrode of transistor Q.sub.5 is connected to the input terminal IN.sub.2. The collector electrode of transistor Q.sub.6 is connected to the commonly connected emitter electrodes of transistors Q.sub.3 and Q.sub.4 whereas the emitter electrode of transistor Q.sub.6 is connected to the collector electrode of transistor Q.sub.8. The base electrode of transistor Q.sub.6 is connected to a source of bias voltage E.sub.2. The emitter electrodes of the transistors Q.sub.7 and Q.sub.8 are grounded respectively through resistors R.sub.4 and R.sub.5, whereas their base electrodes are commonly connected to a source of bias voltage E.sub.3.
FIG. 2 shows in sections (a) to (d) wave forms useful to explain the operation of the modulator system shown in FIG. 1 wherein the waveform of a modulation signal applied to the input terminal IN.sub.2 is shown in section (a), the waveform of a signal to be modulated which is applied to the input terminal IN.sub.1 in section (b), the waveform of the modulated output obtainable at the resistor R.sub.2 in section (c), and the wave form of the modulated output obtainable at the resistor R.sub.1 in section (d), the polarities and the phases of the wave forms shown in FIG. 2 at (c) and (d) being opposite with each other.
The operation of the circuit shown in FIG. 1 will now be described with reference to FIG. 2. The modulation signal applied to the input terminal IN.sub.2 and having a waveform as shown in FIG. 2 at (a) is applied to the base electrode of transistor Q.sub.5 to drive the same. The transistor Q.sub.5 thus driven operates in two manners, that is, (1) as a grounded emitter circuit and (2) as an emitter follower. In the former case (1), the collector output is applied to the commonly connected emitter electrodes of transistors Q.sub.1 and Q.sub.2 for driving the same. At this time, transistors Q.sub.1 and Q.sub.2 connected to cascade with transistor Q.sub.5 are in a grounded base connection and operate so. In the latter case (2) wherein transistor Q.sub.5 is operated in an emitter follower scheme, the emitter output signal thereof will be transmitted to transistor Q.sub.6 via resistor R.sub.3. Since this transistor Q.sub.6 is in grounded base, the modulation signal transmitted through resistor R.sub.3 becomes the collector output of the transistor Q.sub.6 which drives transistors Q.sub.3 and Q.sub.4 with their emitter electrodes commonly connected. At this time, transistors Q.sub.3 and Q.sub.4 are in grounded base and operate so.
The signal to be modulated having a waveform as shown in FIG. 2 at (b) and applied to the input terminal IN.sub.1 drives transistors Q.sub.1 and Q.sub.4. However, as shown in FIG. 1, these transistors Q.sub.1 and Q.sub.4 are emitter coupled with transistors Q.sub.2 and Q.sub.3 respectively so that transistors Q.sub.1 and Q.sub.4 constitute a differential amplifier. The mutual conductance gm of the differential amplifier varies depending upon the driving signal (in this case, the signal to be modulated) so that the differential amplifier will manifest switching characteristics when the driving signal exceeds a predetermined input level. For this reason, when the signal to be modulated, shown in FIG. 2 at (b), assumes a positive half cycle, transistors Q.sub.1 and Q.sub.4 are turned ON, while transistors Q.sub.2 and Q.sub.3 are turned OFF. In the same manner, when the signal to be modulated, shown in FIG. 2 at (b), assumes a negative half cycle, transistors Q.sub.1 and Q.sub.4 are turned OFF, whereas transistors Q.sub.2 and Q.sub.3 are turned ON.
Thus, when transistors Q.sub.1 and Q.sub.3 are alternately turned ON, an output will appear across resistor R.sub.1, whereas when transistors Q.sub.2 and Q.sub.4 are alternately turned ON, an output will appear across the resistor R.sub.2. The wave forms of the outputs are shown in sections (d) and (c) in FIG. 2, respectively. As shown, when transistor Q.sub.1 or Q.sub.2 is ON, the output at the output terminal OUT is such that a modulation signal which is an inversion of the modulation signal as shown in FIG. 2 at (a) is switched with the signal to be modulated shown in FIG. 2 at (b) because at this time the transistor Q.sub.5 is in grounded emitter and operates so. On the other hand, when transistor Q.sub.3 and Q.sub.4 is ON, the output at the output terminal OUT is such that the modulation signal as shown in FIG. 2 at (a) is switched with the signal to be modulated shown in FIG. 2 at (b) because at this time transistor Q.sub.5 operates in an emitter follower scheme, while the transistor Q.sub.6 is in grounded base and operates so.
By repeating the operation described above, a modulated output signal is produced at the output terminal OUT.
With the modulation system, however, since three stages of transistor circuits and two resistors are connected in series between the source and the ground, there are such problems, when the source voltage is low, that the level of the input modulation signal is required to be low, thereby lowering the dynamic level and degrading the signal to noise ratio (S/N) of the modulated output signal with respect to the input signal to be modulated.